The embodiments of the present invention relate to a heating apparatus for use in a vacuum coffee maker, an electric pot capable of boiling water, and so on.
Conventionally, a vacuum coffee maker that uses an alcohol lamp as a heating source for heating a flask has been widely known. A problem in such a vacuum coffee maker is that a difficult operation is required and therefore the coffee maker is inconvenient.
However, prior to the present invention a vacuum coffee maker has been considered which was arranged so that the extraction of coffee essence could be automatically terminated in order to improve the operation of the vacuum coffee maker. FIG. 6 shows an electric circuit of a heating apparatus used in such a vacuum coffee maker.
In FIG. 6, the circuit includes an AC source 51, a main switch 52, a water heater 53 for heating a flask, a heat-retaining heater 54 for the flask, a thermal switch 55 of an automatic reset type which turns on/off in response to a change of temperature at the bottom of the flask, a power-on indicating lamp 56, and an over-temperature preventing device 57. If the main switch 52 is turned on, then the water in the flask is heated by the water heater 53 and the heat-retaining heater 54 so that the water in the flask becomes hot. The hot water is forced up into a funnel by the pressure rising inside the flask so that the flask is heated when no water is contained therein. In this condition, the thermal switch 55 turns off so that the current conduction to the water heater 53 is cut off and the operation mode is changed from the water heating operation to the heat retaining operation. Thereafter, when the temperature decreases inside the flask to thereby lower the inside pressure, the coffee solution in the funnel is drawn back into the flask. The temperature of the coffee solution contained in the flask does not decrease rapidly by heat from the heat-retaining heater 54. The temperature of the coffee solution slowly decreases before finally reaching about 80.degree.-85.degree. C. at which point the coffee solution is subject to a heat-retaining process.
Such a circuit arrangement, however, has various problems in practical use which will be described as follows. For example, in the case when a large volume of coffee is made, it is necessary to successively use the vacuum coffee maker several times because a large volume of coffee making can not be made at one time. In this case, if the on-reset temperature of the thermal switch 55 is set to a low value, the vacuum coffee maker cannot be used for a long period of time before the thermal switch 55 turns on. In contrast the on-reset temperature of the thermal switch 55 is set to a high value, the thermal switch 55 may turn on to carry out the water heating operation again when the funnel is left as it is after making coffee with the funnel fitted in the flask.